Mysterious space plane blasts off for secretive US air force mission

X-37B craft launched from Cape Canaveral on Wednesday, its fourth long orbital flight in five years - but many details of the trip are being kept secret

A mysterious robotic space plane launched its secretive mission for the US air force on Wednesday, its fourth long orbital flight in five years.

The X-37B launched from Cape Canaveral, Florida, for the reusable spacecraft's fourth mission. Its third mission lasted a record-breaking 675 days in space, and ended when it landed at an air force base in California in October 2014.